Chlorine is an important basic material in chemical industry, which is widely used in the industry of new material such as polyurethanes, organosilicones, epoxy resins, chlorinated rubbers, chlorinated polymers, chlorinated hydrocarbons, in industry of new energy such as the manufacturing of polysilicon, in industry of fine chemicals in life such as disinfectors, detergents, food additives, cosmetic additives, in industry of pesticides/pharmaceuticals such as synthetic glycerine, chlorobenzenes, chloroacetic acid, benzyl chloride, PCl3, and in industries of paper making, textile, metallurgy and petroleum chemicals.
In most chlorine-consuming industries, there are two factors which constrain the use of chlorine: 1. in chlorine alkali industry, the electricity consumption for producing one ton chlorine is beyond 2000 kWh; 2. it is hard to deal with the by-product, hydrogen chloride (or hydrochloric acid). A good solution to the two problems is to prepare chlorine by oxidizing hydrogen chloride. According to the development in the recent decades, catalytic oxidation method (Deacon process) is the most effective solution.
In the reported catalysts for the oxidization of hydrogen chloride, active components are mainly metal elements such as copper, chromium, golden and ruthenium, etc. Among them, copper-based catalysts obtain lots of attention because of their lower cost. For example, Chinese patent application CN101125297A discloses a phosphoric acid treated-catalyst containing copper chloride, potassium chloride and cerium chloride, with silica as the carrier. For this catalyst, the yield of chlorine is 80.1% under the conditions that the molar ratio between hydrogen chloride and oxygen is 1:1, the temperature of fixed bed reactor is 400° C., the reaction pressure is 0.1 Mpa and the space velocity of hydrogen chloride is 0.8 hr−1. Higher temperature is required for such catalysts to react, therefore, it is easy to lose the cupric chloride active component and to impair the service life of the catalysts. Chinese patent application CN101559374A discloses a cupric chloride, potassium chloride, manganese nitrate and cerium nitrate loaded catalyst, with silica gel, ReY molecular sieve as carriers, and the conversion of hydrogen chloride is 83.6% under the reaction conditions that the flow rate of hydrogen chloride and oxygen are both 200 ml/min, the amount of catalyst is 25 g and the reaction temperature is 380° C. However, this catalyst still has the disadvantages of loss of copper component, relatively low space velocity and so on. U.S. Pat. No. 4,123,389 discloses a copper-based catalyst with silica gel aluminium oxide or titania as carriers, and the amount of loading of the active component is between 25˜70%. However, the preparation process of said catalysts needs to be carried out in organic solvents, thus resulting in serious pollution to the environment.
However, the hydrogen chloride oxidation process based on copper catalysts has not been industrialized so far, that is because the research of catalysts has not met the requirement of industrialization. Although progresses have been made continuously for hundreds of years, it is still required for the reaction temperature to reach up to 400-450° C. in order to achieve high conversion on copper catalysts. In such a temperature, the chlorides for each component in the catalysts show active mobility, resulting in the lost of components and thus in the deactivation of the catalysts. Chinese Patent application CN102000583A discloses a catalyst prepared by a two-step impregnation method. Said catalyst is a supported catalyst that mainly comprises copper, boron, alkali-metal elements, rare-earth metals and a few other metals, it achieves 85˜89% chlorine yield in a fixed bed reactor, with great stability. However, the disadvantage of said method is: the reaction temperature in the fixed bed reactor is hard to control and temperature runaway is easy to happen, resulting in the lost of the active components of the catalyst by volatilization and influencing the service life of the catalyst and the quality of the chlorine. And because the medium in the reaction process includes hydrogen chloride, chlorine, oxygen, water and so on, which has a strong corrosivity under reaction conditions, only a few expensive nickel based alloys or pure nickel metal materials are suitable for the reactor. Moreover, as the structure of the fixed bed reactor is very complex, the amount of materials required is even larger, resulting in the extreme expensive cost of construction for the tubular fixed bed reactor and the industrialization is not economically sustainable.
As to fluidized bed reactor, since it is convenient to exchange heat and to add or remove a part of catalysts at any time, and there's fewer internal structural components, the problems during actual operation for fixed bed reactors mentioned above can be avoided or relieved. Although for years, internationally, detailed studies have been made to the hydrogen chloride catalytic oxidation process of fluidized bed reactor systems, including Shell-Chlor process, Benson process, Tsinghua University process etc., conditions for industrialization still have not been achieved. We used the composition disclosed in the Chinese patent application CN102000583A to formulate fluidized bed catalyst, and in the fluidized bed reactor, a good reaction effect was achieved initially, however, long-term and stable operation cannot be achieved like in fixed bed reactors. After studying, it has been found that the major reason is that the metal elements in the catalyst formed chloride intermediates by the reaction with hydrogen chloride or chlorine, and these chloride intermediates have melting points that are relatively low, thus they have strong mobility under reaction temperature, even appears to be melted or half-melted. As the reaction time goes on, a part of the active components accumulate on the surface of the catalysts, and the collisions between the catalyst particles might allow the formation of bridging between the particles, thus resulting in the cohesion between the catalysts. The cohesive particles will play a role of crystal nucleus, resulting in the accumulation of more particles and thus in agglomeration of the catalysts, then it will directly affect the fluidization state and condition in the bed, leading to the channeling of the bed, even a dead bed. Meanwhile, partial channeling will lead to temperature runaway of the bed and loss of the active components of the catalysts.
In conclusion, for the chlorine-related industry, HCl oxidation to produce chlorine will be an important step in the industry chain. A good solution to achieve and popularize such a process in a low cost manner is the combination of copper catalysts and a fluidized bed. And in between, the key challenge is to develop a catalyst with good activity, suitable mechanical strength and high stability to adapt the long-term-use requirement in the fluidized bed reactor industrial production.